This invention relates in general to the deposition of polycrystalline silicon and more particularly to the deposition of silicon from a silicon-bearing compound in the presence of a high pressure plasma.
Large quantities of high quality silicon are used by the semiconductor industry for the fabrication of transistors, integrated circuits, solar cells, and the like. The silicon must be of high purity, containing only carefully controlled amounts of conductivity determining dopants. Silicon is typically produced by the hydrogen reduction of a chlorosilane. In the typical process the chlorosilane, usually trichlorosilane, and hydrogen are reacted in a reactor apparatus to deposit pure silicon on a heated filament. The filament can be, for example, either pure silicon or a refractory metal such as tungsten or molybdenum. It is usually about one-half centimeter in diameter and is heated to about 1100.degree. C. by the passage of a heating current. Because the small diameter of the rod provides a heated reaction area of limited size the process starts slowly and is relatively inefficient. Large volumes of gases go unreacted and must be purified for subsequent reuse. Additional large volumes of gases react to form other silicon compounds without resulting in the deposition of silicon. Large quantities of energy are expended, contributing to the high cost of the silicon produced.
A further disadvantage of the prior art process relates to the shape of the outer diameter of the polycrystalline body formed. The rate of deposition and thus the outer shape of the deposited body is partly a function of the temperature of the deposition surface. Any nonuniformities in the deposition filament will result in non-uniformity in the temperature along that filament. This will in turn cause non-uniform deposition rates along the filament and a non-uniform outer diameter.
The filament can also be a source of contaminants. The refractory metal filament can contribute small amounts of unwanted dopants to the depositing silicon at the elevated temperatures used in the deposition process. The filament itself must be bored out of the center of the polycrystalline body before that polycrystalline silicon can be used in subsequent crystal growth processes. This of course entails an additional mechanical operation which is costly and time consuming and which can contribute further contaminants to the silicon.
If a silicon filament is used rather than a refractory metal filament, the silicon must be of extremely high purity, that is, of a purity equivalent to that of the silicon being deposited. Such high purity silicon is, however, very highly resistive and thus difficult to heat uniformly by forcing a current through the filament. The high resistance makes it difficult to force the thousands of Amperes of current through the filament that are required to achieve the deposition temperature.
There is a growing need for high quality, inexpensive polycrystalline silicon resulting from the tremendous present growth in the use of semiconductor products. The availability of low-cost silicon is a necessary prerequisite if silicon photovoltaic cells are to provide an appreciable amount of the country's energy needs. But present methods for producing polycrystalline silicon are expensive, inefficient in the use of both energy and reactants, and tend to yield impure and irregularly shaped silicon deposits. Accordingly a need existed for a silicon deposition process that overcomes these deficiences attendant with present methods.
It is therefore an object of this invention to provide an improved process for harvesting polycrystalline silicon by high pressure plasma deposition.
It is a further object of this invention to provide a process for the deposition of polycrystalline silicon having a high efficiency of input gas utilization.
It is another object of this invention to provide an improved process for the deposition of polycrystalline silicon which is characterized by lower energy consumption than are prior art processes.
It is still another object of this invention to provide a process for the deposition of high purity polycrystalline silicon in shapes having well controlled outer diameters.